(i) Field of the Invention
This invention relates to a control system for 4-wheel drive transmissions to be mounted on vehicles.
(ii) Description of the Prior Art
In combination with a vehicular automatic transmission which is composed of a speed change gear system, friction elements for selectively coupling components of the gear system with an input or output shaft or other components or fixing them to stationary members, and a hydraulic control system for controlling flows of an operating fluid to and from hydraulic servos which operate the friction elements, the 4-wheel drive vehicles generally employ a 4-wheel drive transfer unit which puts the transmission either in a 2-wheel drive mode or in a 4-wheel drive mode with a plural number of gear positions (or reduction ratios). The component parts of the transfer unit are held in position for 2-wheel drive or in one of a number of 4-wheel drive positions by selectively engaging friction elements such as clutches and brakes which are operated by hydraulic servos. In this regard, the conventional 4-wheel drive vehicles are generally designed to engage the friction elements selectively by on-off operations of a plural number of solenoid valves which operate spool valves provided in a transfer control system of the 4-wheel drive automatic transfer unit, which spool valves are located between a pressure source and the hydraulic servos of the clutches and brakes for controlling flows of an operating fluid to and from the hydraulic servos in response to manual operation of a button provided besides a driver's seat. Therefore, difficulties are encountered in that the solenoid valves often malfunction due to electric noises and, under certain operating conditions, an erroneous gearshift is likely to occur in the case of the button type control unless it is confirmed with eyes.